1. Field of the Invention
The present invention is related to a ball bearing to be installed in a spindle motor to be installed in a disc drive device of the magnetic, optical or optimagnetic type such as a hard disc drive device (HDD), a flexible disc drive device (FDD), a digital video disc (DVD), a mini-disc (MD).
2. Related Art of the Invention
For example, the HDD for use in the memory device etc. of the computer has a ball bearing provided around a shaft fixed to the housing fixed to the frame etc. so as to rotatably support a hub. At least one annular hard disc has an inner peripheral edge connected to and supported by the hub to rotate together with the hub. In addition, a stator is fixed to part of the outer peripheral surface of the support shaft, while a motor is fixed to a portion of the inner peripheral surface of the hub facing the outer peripheral surface of the stator whereby a spindle motor is formed to drive and rotate the hub. Power is applied to the stator to drive and rotate the hard disc when using the spindle motor for the HDD.
The ball bearing 1 as shown in FIG. 13 is widely used to be installed in the spindle motor for HDD. The ball bearing 1 comprises an inner ring 3 having an outer peripheral surface formed with an inner ring raceway 2, an outer ring 5 having an inner peripheral surface formed with an outer ring raceway 4 and concentric with the inner ring 3, and a plurality of balls 6 rotatably provided between the inner ring raceway 2 and the outer ring raceway 4. The balls 6 are held rotatably by a cage 8. An annular shield plate 7 is attached to the inner peripheral surface at either end of the outer ring 5. With the shield plate 7 at either end, grease is prevented from leaking to the outside from the portion where the balls 6 are provided, or dust floating in the outside is prevented from entering the portion where the balls 6 are provided.
In the example illustrated, the cage 8 is of the so-called crown type, and as shown in FIGS. 14 to 16, comprises an annular main portion 9 and pockets 10 provided on one axial side (on the upper side in FIG. 16) of the main portion 9 of the main portion 9 and spaced apart from each other with a uniform interval therebetween. Each of the pockets 10 is defined by a pair of resilient piece portions 11 spaced apart from each other with a concave surface portion 12 provided between the pair of resilient piece portions on the one side. The balls 6 are rotatably held in the pockets 10, respectively. The pockets 10 have a generally spherical concave inner surface the radius of curvature of which is slightly larger than the radius of curvature of the rolling contact surface of the balls 6. The cage 8 is integrally made of synthetic resin and formed through injection molding.
The balls 6 are forced in between the pairs of the resilient piece portions 11 resiliently enlarging the distance between the tip ends of the pairs of the resilient piece portions 11 of the pockets 10, respectively. And, as shown in FIG. 17, the balls 6 in the nested state, are rotatably held in the pockets 10, respectively. In this state, there is a minute clearance between the rolling contact surface of the balls 6 and the inner surface of the pockets 10. Accordingly, in the state where the balls 6 are forced in the pockets 10, the balls 6 are rotatably held by the cage 8 with a uniform interval therebetween in the circumferential direction, and the radial position of the cage 8 is restricted by the balls 6. Incidentally, the ball bearing 1 to be installed in the HDD has the inner diameter d3 (FIG. 13) of the inner ring 3 sized between 4 mm to 6 mm.
Recently, the rotation speed of the spindle motor installed in the HDD becomes high as the performance of the personal computer (PC) with the HDD installed therein becomes high. Also, as the density of the HDD becomes high, it is required that the degree of the runout allowed in the hub to which the hard disc is fixed, must be controlled very small. Accordingly, with the ball bearing 1 to be installed in the rotational support portion of the spindle motor, it is required that the rotation resistance is reduced for high rotation, and that the non repeatable runout (NRRO) produced during high speed rotation must be reduced. In addition, it is required that the rotation resistance of the ball bearing 1 is reduced to reduce the power consumption in the rotation machines in which the ball bearing 1 is installed.
Conventionally, however, it was difficult to constantly realize the ball bearings 1 with sufficient performance as mentioned above. The following are reasons of this point.
It is necessary for reduction of the NRRO that minute vibrations must be refrained from occurring in the cage 8 for rotatably holding the balls 6 as the balls 6 rotate as the inner ring 3 and the outer ring 5 rotate relative to each other. If the cage 8 vibrates during operation, the attitude (the location in the axial and circumferential directions of the ball bearing 1) of the balls 6 held in the cage 8 is so unstable to make the NRRO large. In order to suppress the minute vibration of the cage 8 to make the NRRO small, the clearance between the inner surface of the pockets 10 and the rolling contact surface of the balls 6 must be made small to minimize the amount of relative displacement of the cage 8 with reference to the balls 6.
On the other hand, if the clearance between the inner surface of the pockets 10 and the rolling contact surface of the balls 6 is made too small, the rotation resistance of the balls 1 becomes large. Specifically, the clearance is too small, large shearing force exerts to the lubricant film such as grease in the clearance during operation of the ball bearing 1, and the resistance against the spinning balls 6 within the pockets 10 becomes large. As a result, the rotation resistance of the ball bearing 1 is large, it is hard to reduce the power consumption in the HDD etc., in which the ball bearing 1 is installed. Therefore, it is impossible to make the clearance excessively small.
As mentioned above, in order to reduce the NRRO of the ball bearing 1 and to make the rotation resistance small, not only the clearance between the inner surface of the pockets 10 and the rolling contact surface of the balls 6 is made small, but also its size must be properly controlled. The size of the clearance between the inner surface of the pockets 10 and the rolling contact surface of the balls 6 is determined by the pitch circle diameter (PCD) and inner diameter of the cage 8, the inner diameter of the pockets 10, the inner diameter at the opening end edge of the inner surface of the pockets 10, and the outer diameter of the balls 6 and other factors. Accordingly, in the prior art structure, in order to properly control the size of the clearance, the sizes of these factors are separately regulated, and only the cages 8 and balls 6 in which these factors are proper must be selected from the completed ones, and combined with the outer ring 5 and inner ring 3.
However, when the sizes of the factors are separately regulated to make the size of the clearance proper, errors inevitably produced in manufacturing complicatedly effect with respect to the sizes of the factors, and the variation of the size of the clearance may become large. When the variation of the size of the clearance is large, it is difficult to properly regulate the size of the clearance in the ball bearing 1 after it is completed. Accordingly, it was difficult with the prior art structure to constantly manufacture the ball bearing 1 with both of the NRRO and rotation resistance reduced.
Although it is taken into consideration that when the size of the clearance between the inner surface of the pockets 10 and the rolling contact surface of the balls 6 is found to be inadequate, the ball bearing 1 is disassembled and reassembled together with proper parts selected for the proper size of the clearance, it is inconvenient to take such operation for disassembling and reassembling. Accordingly, practically, the ball bearing 1 the clearance size of which is inadequate is thrown away as poor product, and not reused, which increases useless parts, resulting in that the yield of the parts is worsened, and the cost of the ball bearing 1 becomes high.
An objective of the present invention is to provide a structure of the ball bearing in which both of the NRRO and rotation resistance are sufficiently decreased and such ball bearings are constantly manufactured at a lower cost.